Optical fibers have achieved global communication-technology breakthroughs, and enabled high-quality yet largecapacity transoceanic telecommunications. So far, it has been known that a Bragg diffraction grating is prepared in an optical fiber by providing a periodic index profile in a core along the optical fiber. By determining the magnitude of reflectance and the width of frequency characteristics of the diffraction grating depending on the period and length, and the magnitude of refractive index modulation thereof, the diffraction grating is used for wavelength division multiplexers for optical communication purposes, narrow-band high-reflecting mirrors used with lasers or sensors, wavelength selective filters for filtering out extra wavelengths in fiber amplifiers, etc.
However, the wavelength where quartz optical fibers show a minimum attenuation and which is suitable for long-haul communication systems is 1.55 .mu.m. To use an optical fiber diffraction grating at this wavelength, a grating spacing of about 500 nm must be needed. Initially, to make such a fine structure in a core has been considered to be in itself difficult. Accordingly, some complicated process steps comprising side polishing, photoresist step, holography exposure, and reactive ion beam etching are used to make a Bragg diffraction grating in an optical fiber core. Much time is needed for such processes, resulting in limited yields.
In recent years, however, a method of making a diffraction grating by irradiating an optical fiber with ultraviolet radiation for the direct change of a refractive index in a core has been known in the art. This ultraviolet irradiation method has been steadily put to actual use with the progress of peripheral technologies due to no need of complex processes.
This method using ultraviolet light is now carried out by some processes such as an interference process comprising interference of two ray bundles, a writing-per-point process wherein a diffraction grating surface is formed one by one by focusing of a single pulse from an excimer laser), and an irradiation process using a phase mask having a grating, because the grating spacing is as fine as about 500 nm as mentioned above.
The interference process comprising interference of two ray bundles offers a problem in connection with the quality of lateral beams, i.e., spatial coherence, and the writing-per-point process have some operation problems such as the need of submicron careful step control, and the necessity of writing of many surfaces with fine pencils of light.
To address the above problems, an irradiation method using a phase mask has now received attention. As shown in FIG. 7(a), this method uses a phase shift mask 21 obtained by providing grooves on one side of a quartz substrate at a given pitch and a given depth. The phase shift mask 21 is then irradiated with KrF excimer laser light 23 (of 248-nm wavelength) to impart a refractive index change directly to a core 22A of an optical fiber 22, thereby forming a grating. It is here to be noted that reference numeral 22B stands for a cladding of the optical fiber 22. In FIG. 7(a), an interference pattern 24 in the core 22A is illustrated on an enlarged scale for a better illustration thereof. FIG. 7(b) is a sectional view of the phase mask 21, and FIG. 7(c) is a view illustrating a part of the upper surface of the phase mask 21. The phase mask 21 has a binary phase type diffraction grating structure wherein grooves 26, each having a depth D, are provided on one surface thereof at a repetitive pitch P, and a strip 27 having substantially the same width as that of each groove is provided between adjacent grooves 26.
The depth D (a height difference between strip 27 and groove 26) of each groove 26 on the phase mask 21 is selected such that the phase of the excimer laser light (beam) 23 that is the exposure light is modulated by a .pi. radian. Zero-order light (beam) 25A is reduced to 5% or lower by the phase shift ask 21, and primary light (beam) leaving the mask 21 is divided into plus first-order diffracted light 25B including 35% or more of diffracted light and minus first-order diffracted light 25C. By carrying out irradiation using an interference fringe at a given pitch determined by the plus first-order diffracted light 25B and the minus first-order diffracted light 25C, the refractive index change at this pitch is imparted to the core of the optical fiber 22.
The grating in the optical fiber, fabricated using such a phase mask 21 as mentioned above, has a constant pitch, and so the grooves 26 on the phase mask 21 used for grating fabrication, too, have a constant pitch.
Such a phase mask is fabricated by preparing pattern data corresponding to a grating form of groove pitch and carrying out writing with an electron beam writing system to form a grooved grating.
In this regard, a chirped grating wherein the grating pitch increases or decreases linearly or nonlinearly depending on the position of a grating groove in a direction perpendicular to the grating groove (the repetitive direction of grating) is now demanded for the Bragg diffraction grating to be formed in an optical fiber. Such a grating, for instance, is used for high-reflecting mirrors having a widened reflection band, and as delay time control means.
When such a grating having a grating pitch changing linearly or nonlinearly depending on the position of grooves in the lengthwise direction of an optical fiber is fabricated by the interference of plus first-order diffracted light and minus first-order diffracted light using a phase mask, it is required that the pitch of grooves on the phase mask, too, increase or decrease linearly or nonlinearly in a position-dependent manner, as can be seen from the principle shown in FIG. 7(a). The smaller the pitch of grooves on the phase mask, the larger the angle between the plus first-order diffracted light and the minus first-order diffracted light and the smaller the pitch of interference fringes. For the fabrication of such a phase mask with an electron beam writing system, an enormous amount of writing data is needed to write grooves or inter-groove strips all over the range of the mask. This often makes mask fabrication difficult.